Characterization of the Caput Mortuum purple hematite pigment and synthesis of a modern analogue

This investigation addressed the characterization and the production methods of the purple pigmentCaput Mortuum used since pre-historic times until the eighteenth century. Literature hypotheses for its production after heating of ochres/Fe-hydroxides, with addition of white minerals of different composition has been experimentally reproduced, and disproved. An original synthesis after massive hematite that underwent fine grinding, followed by high temperature (900-1100 degrees C) annealing under oxidative conditions was carried out. Several natural and synthetic starting Fe-oxides and hydroxides were tested with these procedures, followed by compositional (X-Ray Powder Diffraction), spectroscopic (Reflectance spectroscopy, colorimetric measurements and Lab CIE color space), microscopic (Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry), and magnetic characterization. The color changes undergone by hematite from red to purple could be mainly traced back to the grain dimensions resulting from the annealing process. The experiments on synthetic starting materials allowed excluding that the final color is affected by impurities. Moreover, it was determined that a purple ochre could not be obtained by thermal treatment of natural earths and that the purple pigment could only be manufactured by starting from hematite and not from red ochre. Finally, magnetic measurements further discriminated among different Caput Mortuum types.

Automated summary

The study experimentally disproved literature hypotheses for the production of purple pigment Caput Mortuum, and found that the final color is mainly affected by the grain dimensions resulting from the annealing process. It was also determined that purple ochre could only be manufactured by starting from hematite and not from red ochre.